A reduction gear train is constituted mainly by four elements: a sun gear driven by the turbine shaft, an annular gear that is coaxial about the sun gear, planet gears meshing with the sun gear and with the annular gear, and a planet carrier.
The reduction ratio in such an assembly is varied by modifying the number of teeth of each of the gears, and by the architecture of the reduction gear train.
There exist two types of configuration:                planetary reduction gear trains in which the planet carrier is stationary and the annular ring is free to rotate; the optimum operating range for this type of reduction gear train corresponds to a reduction ratio lying in the range 1 to 3; and        epicyclic reduction gear trains in which the annular ring is stationary and the planet carrier is free to rotate; the optimum operating range for this type of reduction gear train corresponds to a reduction ratio of 3 or more.        
When the reduction gear train is used for transmitting torque from the gas turbine to a compressor in a turbomachine, the moving element of the reduction gear train, i.e. the annular gear or the planet carrier, is connected to the drive shaft of the compressor, while the other element is connected to the stationary structure of the turbomachine.
Which configuration is selected for a reduction gear train thus depends on the desired reduction ratio. The major drawback of the epicyclic configuration is that in spite of being more compact than a planetary reduction gear train, the planet gears are subjected to a centrifugal acceleration field of the order of 2000 g (where g is the acceleration due to gravity) leading to major difficulties at the bearings for supporting the planet gears.
For a reduction gear train, when considering a planet gear in isolation, it can be seen that the bearing which supports the planet gear is subjected to a radial force equivalent to twice the tangential force generated by meshing. In addition, in an epicyclic reduction gear train, large centrifugal forces are also present.
Since the planet carrier is generally connected to a stationary structure with a planet reduction gear train, or to a transmission shaft with an epicyclic reduction gear train, via one of its front faces, the bearings supporting the planet gears, and more generally the planet carrier, are subjected to twisting torque and will deform, transmitting stresses and deformation to the elements to which the planet carrier is fixed. Such deformation also leads to the gears coming out of alignment and to premature wear of the reduction gear train.
In order to minimize deformation in operation of the planet carrier in an epicyclic reduction gear train, U.S. Pat. No. 5,391,125, which represents the state of the art closest to the invention, proposes a speed reduction gear train for transmitting torque between a gas turbine and a compressor in an aviation turbomachine, which gear comprises a sun gear coaxial with the axis of the reduction gear train and connected to the turbine, a planet carrier connected to the compressor coaxially with the sun gear and having a plurality of planets mounted thereon which mesh with the sun gear, and an annular gear fixed to a structure of the turbomachine and with which the planet gears mesh. In that epicyclic reduction gear train, the planet carrier comprises firstly an annular cage having a plurality of seats for supporting bearings parallel to the axis of the reduction gear train and each having one of the planet gears turning thereon, and a plurality of axial housings, each disposed between two adjacent planet gears, and secondly an annular cage carrier connected to the compressor and having a plurality of axial arms, each arm being disposed in one of the housings of the cage and being fixed to an adjacent portion of the cage by a pin placed in the radial midplane of the cage,
Each pin disposed perpendicularly to the axis of the reduction gear train is inserted in a hole formed in the corresponding arm and in two holes formed radially on either side of the arm in an axial wall of the cage, with shells being interposed therebetween. Such a plug-and-socket type connection does not allow any tilting or axial displacement between the cage and the cage carrier, as can occur in a turbomachine, in particular an aviation turbomachine, in the event of vibration due to turbulence in the ambient air or to thermal expansion.